It is known that manganese dioxide of a grade suitable for use as a depolarizer in dry cell batteries can be manufactured synthetically from precursor manganese sulfate solutions by the oxidation, either electrolytically or by use of an oxidizing agent, of the manganese ion in said solutions to manganese dioxide. Also, it is known that the precursor manganese sulfate solutions can be prepared by the reduction and digestion of naturally occuring manganese ores such as those of the pyrolusite/cryptomelane type. However, a drawback to the use of such manganese ores of the cryptomelane type is the presence therein of high levels of potassium.
The presence of potassium impurity in electrolytic manganese dioxide adversely affects the latter's capability to function satisfactorily as a depolarizer in dry cell batteries. As a result, numerous solutions have been proposed for removing potassium impurity from electrolytic manganese dioxide. The majority of these proposed solutions relate to the removal of potassium impurity during the preparation of the percursor manganese sulfate solution. For example, in U.S. Pat. No. 3,667,906 there is disclosed a method of removing potassium impurity from naturally occurring manganese ores used in the preparation of the manganese sulfate electrolyte comprising washing a reduced or calcined manganese ore with hot water to remove soluble potassium prior to subjecting said reduced or calcined manganese ore to subsequent acid leaching and separation steps.
U.S. Pat. No. 4,285,913 discloses another method for producing manganese sulfate electrolyte solutions containing reduced levels of potassium. In this patent, there is described a process wherein reduced or calcined manganese ore containing potassium impurity is leached with a liquor comprising spent electrolyte (i.e., an aqueous solution containing sulfuric acid and manganese ion) to which has been added a source of iron to provide soluble ferric ions. The leaching process is carried out under specific conditions of pH, time and temperature as to yield a mixture of digested ore and manganese sulfate solution which solution is stated to have reduced levels of potassium impurity. Further, it is stated that subsequent treatment of the mixture with additional reduced or calcined ore to raise the pH of the mixture and to precipitate the remaining iron followed by a liquid/solid separation step thus provides an electrolyte highly suited to the preparation of electrolytic manganese dioxide.
While processes, such as those discussed above, may be effective in reducing the levels of potassium impurity in precursor manganese sulfate solutions, there still remains a need for other, more effective and efficient, processes from an operational and cost standpoint. The present invention provides for such a process.